In a Long Term Evolution (LTE) mobile communications system, as user equipment (UE) moves, a network hands over the UE from a source cell to a target cell by using a handover process, to perform data transmission.
After determining to perform handover, a source base station sends a handover command to the UE at an air interface, where the message is a radio resource control (RRC) connection reconfiguration message that carries a mobility control information element (mobilityControlInfo), and stops uplink and downlink data transmission performed on the UE. After receiving the handover command, the UE performs synchronization in a target cell to which the UE is handed over, and initiates a random access process to obtain a timing advance (TA) value and an uplink resource. The UE sends, on a corresponding uplink resource, an RRC connection reconfiguration completion message to a target base station, to indicate completion of the handover. After receiving the RRC connection reconfiguration completion message, the target base station recovers the uplink and downlink data transmission performed on the UE.
It can be learned from the foregoing description about the handover process that, from a time when the source base station sends the handover command to the UE to a time when the target base station receives the handover completion indication, the uplink and downlink data transmission performed on the UE is interrupted. This severely affects service experience for a delay-sensitive service, and greatly limits application of the delay-sensitive service in the LTE system, and in particular, application in a mobile scenario.
With development of a mobile communications system, service quality that can be provided by the system is becoming higher. To maintain long-term competitive edge of the 3rd Generation Partnership Project (3GPP), and further increase spectral efficiency of the system and a user throughput rate, an LTE-Advanced (LTE-A) standard is formulated. Carrier aggregation (CA) is introduced as a new technology into the LTE-A standard. The carrier aggregation means that UE may perform uplink and downlink communication by simultaneously using a plurality of cells (carriers), thereby supporting high-speed data transmission. Among the plurality of cells, one is a primary cell (PCell), and the rest is a secondary cell (SCell).
To further increase the spectral efficiency of the system and the user throughput rate, the 3GPP further introduces a dual connectivity (DC) technology into the LTE-A standard, that is, supports two base stations to simultaneously provide a data transmission service to one UE. A base station that the PCell belongs to is referred to as a master eNodeB (MeNB), and the other base station is referred to as a secondary eNodeB (SeNB). In DC, a plurality of serving cells served by the MeNB form a primary cell group (MCG), and include one PCell and one or more optional SCells. A plurality of serving cells served by the SeNB form a secondary cell group (SCG).
For a DC scenario, there are three types of data bearers: an MCG bearer, an SCG bearer, and a split bearer. When establishing a data bearer, the network designates a specific bearer type. Data of the MCG bearer can be transmitted by using only a serving cell served by the MeNB, and encrypted or decrypted in the MeNB, and uplink data is sent to a serving gateway (S-GW) using the MeNB or downlink data is received from an S-GW by using the MeNB. Data of the SCG bearer can be transmitted by using only a serving cell served by the SeNB, and encrypted or decrypted in the SeNB, and uplink data is sent to an S-GW by using the SeNB or downlink data is received from an S-GW by using the SeNB. Data of the split bearer can be transmitted by using a serving cell served by the MeNB or the SeNB, but can be encrypted or decrypted only in the MeNB, and uplink data is sent to an S-GW by using the MeNB or downlink data is received from an S-GW by using the MeNB.
When the network adds an SeNB for UE, the MeNB derives a new key based on a key of the MeNB and sends the new key to the SeNB for use, and the MeNB also sends the derived parameter to the UE. The UE derives a same key based on these derived parameters.
According to the current 3GPP protocol, for a single carrier scenario, a CA scenario, or a DC scenario, when a PCell of UE is changed, the LTE system triggers a handover process, and data transmission of the UE is interrupted in the handover process. The single carrier scenario means that the UE is in a single carrier mode, and the UE communicates with the network by using only one serving cell. The CA scenario means that the UE is configured to be in a CA mode, and the UE communicates with the network by using at least two cells served by one base station. The DC scenario means that the UE is configured to be in a DC mode, and the UE communicates with the network by using a plurality of cells served by two base stations. As network deployment density increases, a movement of UE causes the PCell to be frequently changed. Consequently, data transmission of the UE is frequently interrupted, severely affecting service experience of an end user, and in particular, service experience for a delay-sensitive service.